The present invention relates to an apparatus for recording information, and more particularly, it relates to an apparatus for recording digital signal information.
Structures of the type characterized by employing a PRML (Partial Response Maximum Likelihood) method to read a signal magnetically recorded have been disclosed in, for example, Japanese Patent Laid-Open No. 5-2842 and Japanese Patent Laid-Open No. 4-221464. A known example of the type using the PRML to record information magneto-optically has been disclosed in "Novel Application of Viterbi Decoding Method Utilizing Variable-Length Block Codes and Limitation d to Magneto-Optical Recording", the Television, 44, 10, pp. 1369 to 1375 written by Osawa, Yamauchi and Tazaki (1990). Since the OTF (Optical Transfer Function) of the optical system for use in magneto-optical recording is a SINC function, a PR system called "PR (1,1)" having frequency characteristics like those of the SINC function has been employed. The system in the foregoing case is, for example, shown in FIG. 2. A modulator 21 modulates recording data and a precoder 22 precodes the modulated recording data to obtain a code. The code is then recorded and read by a magneto-optical drive 23. The waveform of a read-out signal is changed by an equalizer 24, and noise of the read-out signal is removed by a low pass filter (LPF) 25. Then, an output signal from LPF 25 is quantized by an A/D converter 26. The thus-obtained quantized value is used for decoding in a Viterbi decoder 27 before it is demodulated by a demodulator 28. The cut-off frequency of LPF 25 is usually set to 1/2 D in the above case, D being the delay time for one bit of the read-out signal. Further, the characteristic of precoder 22 is made to be [1/(1+D)] and the characteristics obtained by adding the characteristic of magneto-optical drive 23 and that of equalizer 24 are made to be (1+D) so that a read-out signal, the recording/reading characteristics of which have been canceled, is supplied to Viterbi decoder 27. Equalizer 24 has a structure, for example, as shown in FIG. 3. Equalizer 24 comprises delay devices 31, coefficient multipliers 32 and an adder 33. Viterbi decoder 27 has a presupposed path memory 41 (shown in FIG. 4) storing expected values obtained from the waveforms of data columns having number of bits corresponding to the constraint length. An ACS circuit 42 includes an adder (A), a comparator (C) and a selector (S). By the adder (A), ACS circuit 42 obtains the sum of the square output of the difference between a sample value of the read-out signal, the waveform of which has been equalized by an equalizer 24, and the expected value supplied from presupposed path memory 41 and a path metric value calculated previously. The outputs representing the results of the additions are subjected to a comparison by the comparator (C) to selectively output the smaller output from the selector (S). The final value of the thus-selected presupposed path memory is supplied to a path memory 43. Although the value received by path memory 43 is not the most probable value as the demodulated data, it is the most probable value at present because it continues to the presupposed path. A pass selector 44 selects the minimum value of the path metric value at the above moment to select the path continuing to the above state to make the final data to be the demodulated data.
The conventional system of the type combining PR(1, 1) and Viterbi decoding is an excellent detection method because of its satisfactory decoding capability as compared with the level detection system. The problems experienced with PR (1, 1) will now be described while describing the method of designing the equalizer and so forth for use in the above-described detection system. In a case of FIG. 2, assuming that the transfer function of magneto-optical drive 23 is H (f), the transfer function of equalizer 24 is E (f), and the transfer function of PR (1, 1) ((1+D)) is PR.sub.11 (f). Equalizer 24 is designed to satisfy the following equation: EQU H(f).multidot.E(f)=PR.sub.11 (f)
FIG. 5 is a graph showing the frequency characteristic of the transfer function of equalizer 24 expressed by a curve 51, the frequency characteristic of the transfer function of ideal PR (1, 1) equalization expressed by a curve 52 and the frequency characteristic of the transfer function of magneto-optical drive 23 expressed by a curve 53 in a case where the recording density is low. In the case of FIG. 5, frequency f.sub.11 that realizes the equation PR.sub.11 (f)=0 is lower than frequency f.sub.H that realizes the relationship H(f)=0. Therefore, equation H (f).multidot.E(f)=PR.sub.11 (f) is substantially accurately held. In this case, equalization error can be decreased relatively. If the recording density shown in FIG. 5 is raised, the value of f.sub.11 is enlarged, resulting in f.sub.11 =f.sub.H and then f.sub.11 &gt;f.sub.H. FIG. 6 shows the frequency characteristic of the transfer function of equalizer 24 expressed by a curve 61, the frequency characteristic of the transfer function of ideal PR (1, 1) equalization expressed by a curve 62 and the frequency characteristic of the transfer function of magneto-optical drive 23 expressed by a curve 63 in a case where the relationship f.sub.11 &gt;f.sub.H is realized. In the foregoing case, a state H(f)=0 is held in the state where the frequency holds the relationship f.sub.H &lt;f. As a result, the relationship H(f).multidot.E(f)=PR.sub.11 (f) cannot be held. Therefore, it is mathematically impossible to design E (f) that completely satisfies H (f).multidot.E (f)=PR.sub.11 (f). As a result, there arises a problem that the equalization error becomes more critical. In order to prevent the equalization error, the number of taps of the transversal filter must be increased. Even if the number of the taps is increased, there arises a problem in that the equalization error cannot satisfactorily be prevented, resulting in unexpected intensification of high frequency exaggeration. As a result, there arises problems in that the equalizing performance of PR (1, 1) deteriorates and the decoding performance also deteriorates.